Workers Development

Workers Development

Purpose

This skill provides comprehensive guidance for developing Cloudflare Workers, including runtime APIs, fetch event handlers, request/response handling, bindings usage, and common development patterns. Use this skill when implementing Workers code, designing Workers architecture, or working with the Workers runtime environment.

Workers Runtime Overview

Cloudflare Workers run on the V8 JavaScript engine at the edge. Workers use the Service Worker API with extensions specific to the Cloudflare platform.

Key Characteristics

• Isolate-based architecture: Each request runs in an isolated V8 context, not a container
• Fast cold starts: Sub-millisecond startup time due to isolate architecture
• Automatic scaling: No configuration needed, scales to millions of requests
• Global deployment: Code runs at 300+ Cloudflare data centers worldwide
• Standards-based: Uses Web APIs (fetch, Request, Response, Headers, etc.)

Execution Model

Workers execute on incoming requests:

1. Request arrives at Cloudflare edge
2. Worker isolate spawns (or reuses existing)
3. Fetch event handler executes
4. Response returns to client
5. Isolate may persist for subsequent requests

Fetch Event Handler

The fetch event handler is the entry point for Workers:

export default {
  async fetch(request, env, ctx) {
    // request: Request object
    // env: Bindings and environment variables
    // ctx: Execution context with waitUntil() and passThroughOnException()

    return new Response('Hello World');
  }
};

Parameters

request: Incoming Request object (Web API standard)

• request.url - Full URL string
• request.method - HTTP method (GET, POST, etc.)
• request.headers - Headers object
• request.body - ReadableStream of request body
• request.cf - Cloudflare-specific properties

env: Environment object containing bindings

• KV namespaces: env.MY_KV
• D1 databases: env.MY_DB
• R2 buckets: env.MY_BUCKET
• Durable Objects: env.MY_DO
• Secrets: env.MY_SECRET
• Environment variables: env.MY_VAR

ctx: Execution context

• ctx.waitUntil(promise) - Extend execution lifetime for async tasks
• ctx.passThroughOnException() - Pass request to origin if Worker throws

Return Value

Must return a Response object or a Promise that resolves to a Response:

// Direct return
return new Response('Hello', { status: 200 });

// Async return
return await fetch('https://api.example.com');

// With headers
return new Response(JSON.stringify({ ok: true }), {
  status: 200,
  headers: {
    'Content-Type': 'application/json'
  }
});

Request Handling Patterns

Routing

Common routing patterns for multi-route Workers:

export default {
  async fetch(request, env, ctx) {
    const url = new URL(request.url);

    // Path-based routing
    if (url.pathname === '/api/users') {
      return handleUsers(request, env);
    }

    if (url.pathname.startsWith('/api/')) {
      return handleAPI(request, env);
    }

    // Method-based routing
    if (request.method === 'POST') {
      return handlePost(request, env);
    }

    // Default
    return new Response('Not Found', { status: 404 });
  }
};

See examples/fetch-handler-patterns.js for complete routing examples.

Request Inspection

Access request properties:

const url = new URL(request.url);
const method = request.method;
const headers = request.headers.get('Authorization');
const cookies = request.headers.get('Cookie');

// Cloudflare-specific properties
const country = request.cf?.country;
const colo = request.cf?.colo; // Data center code

Request Body Parsing

Parse request bodies based on content type:

// JSON
const data = await request.json();

// Form data
const formData = await request.formData();
const field = formData.get('fieldName');

// Text
const text = await request.text();

// Array buffer
const buffer = await request.arrayBuffer();

Response Construction

Basic Responses

// Text response
return new Response('Hello World');

// JSON response
return new Response(JSON.stringify({ message: 'Success' }), {
  headers: { 'Content-Type': 'application/json' }
});

// HTML response
return new Response('<h1>Hello</h1>', {
  headers: { 'Content-Type': 'text/html' }
});

// Status codes
return new Response('Not Found', { status: 404 });
return new Response('Created', { status: 201 });

Headers

// Set headers
const headers = new Headers({
  'Content-Type': 'application/json',
  'Cache-Control': 'max-age=3600',
  'X-Custom-Header': 'value'
});

return new Response(body, { headers });

// CORS headers
const corsHeaders = {
  'Access-Control-Allow-Origin': '*',
  'Access-Control-Allow-Methods': 'GET, POST, OPTIONS',
  'Access-Control-Allow-Headers': 'Content-Type'
};

Redirects

// 301 permanent redirect
return Response.redirect('https://example.com', 301);

// 302 temporary redirect
return Response.redirect('https://example.com', 302);

Bindings

Bindings provide access to Cloudflare resources and are configured in wrangler.toml or wrangler.jsonc.

Binding Types Overview

• KV: Key-value storage for static content
• D1: SQLite database for relational data
• R2: Object storage for large files
• Durable Objects: Stateful coordination and real-time features
• Queues: Message queuing for async processing
• Vectorize: Vector database for embeddings
• Workers AI: AI inference and embeddings
• Service bindings: Call other Workers
• Environment variables: Configuration values
• Secrets: Sensitive credentials

See references/bindings-guide.md for complete binding configuration and usage patterns.

Common Binding Usage

KV (Key-Value):

// Read
const value = await env.MY_KV.get('key');
const json = await env.MY_KV.get('key', 'json');

// Write
await env.MY_KV.put('key', 'value');
await env.MY_KV.put('key', JSON.stringify(data), {
  expirationTtl: 3600 // Expire in 1 hour
});

D1 (Database):

// Query
const result = await env.MY_DB.prepare(
  'SELECT * FROM users WHERE id = ?'
).bind(userId).first();

// Insert
await env.MY_DB.prepare(
  'INSERT INTO users (name, email) VALUES (?, ?)'
).bind(name, email).run();

R2 (Object Storage):

// Read
const object = await env.MY_BUCKET.get('file.txt');
const text = await object.text();

// Write
await env.MY_BUCKET.put('file.txt', 'content');

Error Handling

Try-Catch Patterns

export default {
  async fetch(request, env, ctx) {
    try {
      // Your logic here
      const result = await processRequest(request, env);
      return new Response(JSON.stringify(result), {
        headers: { 'Content-Type': 'application/json' }
      });
    } catch (error) {
      console.error('Error:', error);
      return new Response(JSON.stringify({
        error: error.message
      }), {
        status: 500,
        headers: { 'Content-Type': 'application/json' }
      });
    }
  }
};

Error Response Helpers

function errorResponse(message, status = 500) {
  return new Response(JSON.stringify({ error: message }), {
    status,
    headers: { 'Content-Type': 'application/json' }
  });
}

// Usage
if (!apiKey) {
  return errorResponse('API key required', 401);
}

See examples/error-handling.js for comprehensive error handling patterns.

Async Operations with waitUntil

Use ctx.waitUntil() to perform background tasks that extend beyond the response:

export default {
  async fetch(request, env, ctx) {
    // Respond immediately
    const response = new Response('Request received');

    // Continue processing in background
    ctx.waitUntil(
      logRequest(request, env)
    );

    return response;
  }
};

async function logRequest(request, env) {
  await en